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Effects of Grazing. (View larger image)
Credit: F.H. Maude
Learning
About SW Climate > Overview of Fire Research
Effects
of Grazing
Numerous fire and land use histories in the Southwest indicate that intense livestock grazing was the initial cause of decline in the natural pattern of frequent, low-intensity fires at regional and local scales in the late 1800s (Leopold 1924, Swetnam and Baisan 1996). Beginning in the 1880s, large numbers of cattle and sheep were introduced into southwestern forests. As grazers consumed the grasses and other herbaceous vegetation that fires need in order to spread, fire activity dropped off. In addition, the trails created by livestock over time probably constrained the spread of fire as well by breaking up the continuity of the surface fuels.
As grasses were reduced and fires ceased to spread across the landscape, more trees were able to get established. The effects of grazing were then multiplied when fire suppression became the norm early this century. Eventually, open areas were replaced by dense tree stands. According to Dr. Craig Allen of the USGS, historically, a typical density of ponderosa pines in the Southwest was around 100 stems per acre. Today, densities at many sites exceed 2,000 stems per acre. When fires do get established, they now often leave the ground and climb ladder fuels (brush, shrubs, standing snags, and mid-sized stems) into the treetops. Extensive crown fires in Southwest ponderosa pine forests were not seen until around the 1950s, as it took that long for the forests to get dense enough and for the fuel conditions to change.
References
Leopold, A., 1924: Grass, brush, timber and fire in southern Arizona. Journal of Forestry 22:1-10.
Swetnam, T.W., and C.H. Baisan. 1996: Historical fire regime patterns in the southwestern United States since AD 1700. In C. D. Allen, editor, Fire effects in southwestern forests, Proceedings of the Second La Mesa Fire Symposium, March 29-31, 1994, Los Alamos, New Mexico. U.S. Department of Agriculture U.S. Forest Service General Technical Report RM-GTR-286. 216 pp.
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